This invention relates to intake shields for terrestrial and marine gas turbine engines and is concerned with the provision of an intake shield to be used while operating stationary gas turbine engines in order to protect them from the ingestion of debris and vortices and to reduce the level of noise generated while carrying out engine testing procedures.
Gas turbine engines consume copious quantities of air, which makes the operation of stationary engines difficult due to the ingestion of vortices. Such vortices are formed by the interaction of the entrained air with solid surfaces anywhere within the vicinity of the engine. The strengths of such vortices can be intensified by winds coming either from the sides or to the rear of the engine intake. Ingested vortices lead to excessive noise and engine instability and can cause mechanical damage to the engine.
A particularly troublesome example of vortex ingestion occurs when aircraft engines undergo ground run-up testing following maintenance work. Numerous complaints about excessive noise are currently received by airport operators and airlines. Furthermore, testing often has to be postponed when the wind is in an unfavourable direction because of the instability of the engine.
During present testing procedures, it is common practice to locate a debris guard around the engine intake as a protection for personnel and to prevent ingestion of debris into the engine, where it could cause damage. These debris guards are portable and partly enclose a rectangular space. They comprise a base, three sides which extend in height to approximately the centre of the engine, and a fourth side, which is of lower height so that it can pass underneath the engine. No tops are provided for these guards and their walls and bases are typically constructed from sturdy wire fences held in position by steel frames, the bases typically being attached to a chassis mounted on wheels.
Although the existing debris guards protect the engine from debris and form a personnel barrier, they do not protect the engine from the effect of vortices.
It is also known to provide special intake shields which are used to assess the noise level of new engines. These have been employed since research showed that high by-pass ratio aircraft engines are at least 10 decibels quieter when in flight than when running stationary close to the ground. Engine manufacturers make allowance for this phenomenon during noise surveys by mounting the engine rigidly on a high stand in order to reduce ingested vorticity, and fit a special bell-mouth engine intake to provide a uniform stream of air into the engine. To further reduce vorticity and turbulence, this intake draws its air through a very large spherical porous screen with the engine intake located close to its centre. The diameter of such a screen is many times greater than that of the engine, but the screens are fragile, expensive and too large to be suitable for on-wing testing of engines after maintenance. From the nature of their application, these spherical screens are designed not to attenuate the sound radiated from the engine but just to reduce the level of turbulence.
It is accordingly an object of the present invention to provide an intake shield which can be used to protect an engine from both debris and vortices while it is being tested close to the ground.
U.S. Pat. No. 5,411,224 (Dearman et al.) discloses a jet engine intake guard which includes a generally circular base ring member for connection to an engine cowl of a jet engine.
U.S. Pat. No. 3,871,844 (Calvin) discloses a screen apparatus for the air inlet of a jet turbine engine which is designed to prevent foreign objects from entering the air inlet.
U.S. Pat. No. 5,591,904 (Schafhaupt et al.) discloses an apparatus for use in diminishing intake vortexes at jet engines during testing which consists of a grating disposed near the ground below and directly in front of the engine inlet.
British Patent Specification No. 2,076,102 relates to a debris guard for mounting across the air intake of a gas turbine engine.
It is a further object of the present invention to provide an intake shield for a gas turbine engine air intake which provides improvements over the prior art devices disclosed in the specifications referred to above.
According to the present invention there is provided an intake shield for a gas turbine engine air intake, said shield comprising a wheeled structure containing a hollow chamber which locates over the engine intake so as to totally enclose the engine intake apart from an aperture through which the engine intake is inserted, said chamber having at least one wall which is formed with a plurality of flow passages so as to be porous to air but impervious to vortices and debris, and the dimensions of the chamber being such that, in use, the engine draws its air through the passages in said at least one wall which present a greater area for the flow of air than that of the engine intake.
The dimensions of the chamber may be such that the full quantity of air required by the engine can be drawn through the walls of the chamber without incurring a significant pressure loss.
One or more of the following features may also be provided:
a) means for adjusting the height of the shield to facilitate its location over the engine intake,
b) means for adjusting the size and position of the aperture through which the engine intake is located,
c) means for sealing the gap between the aperture and the engine,
d) the sealing means are retractable,
e) the wheels of said structure are on a wheeled chassis,
f) the porous walls are arranged to direct the airflow towards the engine,
g) the porosity is graded to produce a uniformity of airflow entering the engine,
h) the porous walls are contoured to provide a uniformity of airflow entering the engine,
i) part of a wall facing the engine is made of acoustic absorbing material,
j) a settling length between the porous wall(s) and the engine intake in order for any turbulence or non-uniformity in the flow of air to settle,
k) a nozzle to guide the air stream smoothly into the engine intake,
l) guiding means for assisting guidance of the shield while locating it over the engine intake,
m) a telescopic attachment to simplify coupling of the shield to the engine, and
n) an anchor attachment for transmission of the thrust acting on the shield either to the ground or to an exhaust device located behind the engine.